Advantages of high efficiency and high torque at low speed are realized by rapid near-top-dead-center injection of fuel jets. Such direct injection creates its own turbulence, burning at a characteristically turbulence-limited combustion rates. Diesel engines operate using this strategy.
A disadvantage of conventional diesel engines is the tendency to produce soot as a result of incomplete oxidation of the fuel. In this disclosure, the term “soot” includes particulate matter generally. Another tendency is to produce excessive nitrogen oxides (NOx) due to diffusion combustion—the mode by which directly injected diesel fuel burns. Diffusion combustion takes place near the stoichiometric temperature. Higher temperature tends to form more NOx.
Further, it has long been known that measures that tend to reduce the production of NOx also tend to increase the production of soot and vice versa. However, if soot production could be inhibited, the production of NOx could also be reduced. Moreover, soot reduction should also increase power density.
A related issue that impacts on soot formation and the flexibility of managing NOx in diesel engines relates to partial re-circulation of exhaust gas (EGR). EGR helps to reduce combustion temperature and, as a result, NOx. The reduction of combustion temperature, however, adversely impacts soot formation. Further, experimental measurements indicate that EGR also tends to reduce burning rate. Incomplete combustion resulting from EGR reduces the efficiency of the engine overall. Therefore, as soot production and incomplete combustion limit the extent to which EGR can be utilized, the advantages of increasing the combustion rate and the soot oxidation rate are apparent.
Developments in engine technology have shown that diesel engines can be fuelled by gaseous fuels. Some of these developments show that this can be done with no real impact on power and/or efficiency. Examples of such gaseous fuels include natural gas, methane, propane, ethane, gaseous combustible hydrocarbon derivatives and hydrogen.
Natural gas will be discussed in the context of this disclosure however, as would be understood by a person skilled in the art, the other gaseous fuels noted may be adapted. Substituting diesel with such natural gas results in emissions benefits over diesel. Specifically, lower NOx and soot levels are found in the exhaust gas.
A method used to ensure that natural gas matches, for the most part, the power and efficiency found in diesel-fueled ignition engines, relies on high-pressure direct injection followed by diffusion combustion. That is, natural gas is directly injected at high pressure into a combustion chamber where an ignition source is usually used to ignite the natural gas. Due to such direct injection and diffusion combustion, this fuel generally suffers from the same issues noted above in regards to soot and NOx generation, albeit at significantly lower levels than is the case with diesel fuel. The same zone of incomplete oxidation found in regards to combustion resulting from diesel-fuelled compression ignition strategies is thought to result. As such, while natural gas provide a significant reduction of particulates and NOx, these fuels, directly injected, are governed by some of the same physical processes found in diesel-fuelled engines. Therefore, room is available to manage soot and particulate production in both natural gas and diesel-fuelled direct injection engines.
Dec, J. E., “A Conceptual Model of DI Diesel Combustion based on Laser-Sheet Imaging”, SAE 970873, 1997 provided a physical understanding of conventional diesel fuel combustion for the quasi-steady period of burning. It appears that combustion takes place in two phases. The first occurs in the rich mixture created by entrainment of air into the fuel jet. Here the equivalence ratio is so high that the flame temperature is low (perhaps around 1600° K) and soot forms by pyrolysis due to the shortage of oxygen. A soot-rich zone is created that is surrounded by a thin region in which final mixing and any remaining chemical reactions occur. Understanding the behavior of this soot rich zone provides a starting point for reducing the production of soot in the diesel and natural gas engines described above.
Sjoeberg, in “The Rotating Injector as a Tool for Exploring DI Diesel Combustion and Emissions Formation Processes”, 2001, ISSN 1400-1179, provided a rotating injector that, in effect, caused turbulence that impacted on the soot-rich zone by moving the fuel jet throughout the combustion chamber. Such a strategy, however, is difficult to implement. A rotating injector introduces moving parts to the engine that are susceptible to wear and durability issues.
The present invention deals with the above noted problems related to directly injected fuels used in internal combustion engines.